Semiconductor Porous Hydrogen-Bonded Organic Frameworks Based on Tetrathiafulvalene Derivatives

Herein, we report on the use of tetrathiavulvalene-tetrabenzoic acid, H4TTFTB, to engender semiconductivity in porous hydrogen-bonded organic frameworks (HOFs). By tuning the synthetic conditions, three different polymorphs have been obtained, denoted MUV-20a, MUV-20b, and MUV-21, all of them presenting open structures (22, 15, and 27%, respectively) and suitable TTF stacking for efficient orbital overlap. Whereas MUV-21 collapses during the activation process, MUV-20a and MUV-20b offer high stability evacuation, with a CO2 sorption capacity of 1.91 and 1.71 mmol g–1, respectively, at 10 °C and 6 bar. Interestingly, both MUV-20a and MUV-20b present a zwitterionic character with a positively charged TTF core and a negatively charged carboxylate group. First-principles calculations predict the emergence of remarkable charge transport by means of a through-space hopping mechanism fostered by an efficient TTF π–π stacking and the spontaneous formation of persistent charge carriers in the form of radical TTF•+ units. Transport measurements confirm the efficient charge transport in zwitterionic MUV-20a and MUV-20b with no need for postsynthetic treatment (e.g., electrochemical oxidation or doping), demonstrating the semiconductor nature of these HOFs with record experimental conductivities of 6.07 × 10–7 (MUV-20a) and 1.35 × 10–6 S cm–1 (MUV-20b).


■ INTRODUCTION
The design of materials combining electrical conductivity and porosity is an interesting but challenging topic that has gained exponential attention in recent years for exploitation in nextgeneration applications. 1,2 Despite being usually considered insulating materials, metal−organic frameworks (MOFs) 3 and covalent organic frameworks (COFs) 4 are porous crystalline structures that have proven to be efficient for the transport of electrical charge with a suitable chemical design. 5−7 This has been achieved through organic linkers with an extended πconjugation, 8,9 via the existence of a π−π stacking throughspace pathway, 10−12 or with the incorporation of electroactive guest molecules within the pores of these crystalline materials. 13 Most relevant strategies to boost conductivity in porous materials rely on charge carrier formation, for example, through partial ligand oxidation upon iodine doping or partial metal-node oxidation/reduction in mixed-valence frameworks. 14 However, these approaches imply either a trade-off where porosity and available surface area are decreased or a very limited versatility to access mixed-valence states (e.g., Fe(II)/Fe(III)-based frameworks), respectively. Moreover, the implementation of MOFs/COFs in efficient devices is not trivial, 6 although different approaches have been successfully accomplished in this regard. 15−18 Hydrogen-bonded organic frameworks, or HOFs, 19−21 have appeared as an alternative type of porous molecular-based crystalline materials that are self-assembled through H-bonding interactions. Similar to MOFs and COFs, HOFs have been used in gas storage, 22 separation, 23 encapsulation, 24 or proton conductivity, 25 among others. 26 However, due to the lack of formation of strong coordination bonds (as in MOFs) or covalent bonds (as in COFs), HOFs can be synthesized under mild conditions, 27 which facilitates their processing, one of the major drawbacks of MOFs and COFs. 16,20 Thus, the preparation of conductive HOFs appears as an intuitive solution for the problematic synthetic methods available for MOFs and COFs.
Tetrathiafulvalene (TTF) derivatives are a well-known family of molecular-based conductors extensively studied in the field of molecular electronics, 28,29 as well as for being used in the preparation of conductive MOFs 30 and COFs. 31 In these systems, in addition to a suitable orbital overlap, it is essential to obtain the radical cation TTF •+ to engender the charge carrier species. This usually implies the oxidation of the material, typically with I 2 or Br 2 , which results in a decreased sorption capacity due to the incorporation of the reduced I 3 − or Br − species. 32 Tetrathiafulvalene-tetrabenzoic acid, in short H 4 TTFTB, is a well-known ligand that has been used by us 33−37 and others 38,39 in the preparation of MOFs and COFs. This ligand has also been employed in the construction of a HOF with limited stability, which collapses upon desolvation, 40 and more recently, it has shown to be involved in single-crystal-to-singlecrystal transformations. 41 A very recent development in TTFbased HOFs has been shown by Farha and co-workers, who have prepared a HOF with a naphthalene derivative of TTF, which, upon oxidation with I 2 , becomes a semiconductor (conductivity of 6.0 × 10 −7 S cm −1 ). 42 Herein, we present the use of the H 4 TTFTB molecule to prepare two new porous HOFs, in which the building block ligand has a zwitterionic character. These materials, denoted MUV-20a and MUV-20b, do not require additional doping for electrical charge transport, resulting in two porous conducting HOFs while preserving their crystalline porosity. The semiconducting properties of MUV-20a and MUV-20b stand record within the HOF materials and contrast with those exhibited by a non-zwitterionic HOF based on the same ligand, denoted MUV-21, which behaves as an insulator despite the suitable stacking of the TTF units. Zwitterionic polaron formation within an organic crystal therefore emerges as an new potential strategy to boost conductivity in porous materials.

■ RESULTS AND DISCUSSION
TTF-tetrabenzoic ether was prepared from the coupling of TTF and ethyl 4-bromobenzoate, followed by hydrolysis and acidification (see Section S1 in the Supporting Information). Importantly, acidification resulted in the protonation of three among the four carboxylate groups, resulting in an amorphous material of formula NaH 3 TTFTB, as seen with 1 H NMR (see Figure S1) and energy-dispersive X-ray analysis (EDAX) (see Figure S2 and Table S1). Dissolution of NaH 3 TTFTB in THF and heating at 80°C forms a transparent dark-red solution, and addition of diethyl ether causes the appearance of red needle-like crystals of MUV-20a after few minutes at room temperature ( Figure 1). Upon washing the crystals with diethyl ether, the THF molecules are replaced by diethyl ether molecules, yielding crystals of MUV-20b ( Figure 2). These crystals were suitable for single-crystal X-ray diffraction analysis (see Table S2 for crystallographic details). Quite differently, crystallization from DMF at 105°C resulted in large single crystals of MUV-21 ( Figure 3 and Table S2). Phase purity of each sample was confirmed by powder X-ray diffraction ( Figures S28−S30).
MUV-20a forms a 2D hydrogen-bonded network involving three carboxylic acids from each molecule (Figure 1a), with O···O hydrogen-bond distances in the range of 2.60−2.73 Å ( Figure S3), forming voids with dimensions of ca. 7 × 9 Å ( Figure S4). The 2D layers are interconnected via π−π stacking between the benzene moieties possessing the carboxylate group that is not involved in hydrogen bonding (Figure 1b), with a distance of 3.28 Å between parallel benzene groups ( Figure S5). As a result, interpenetration between adjacent layers occurs (blue and red in Figure 1d). The stacking of the layers results in a 3D-networked, rhombic, interpenetrated framework. The structural features of MUV-20a are completed with π−π stacking between the TTF moieties (Figures 1c, S6, and S7), with shortest S···S distances of 3.66 and 4.45 Å for closest and furthest neighbors, respectively (Tables S3 and S4).
In contrast, structural analysis of MUV-21 ( Figure 3) reveals that the three carboxylic acids and the carboxylate group are involved in hydrogen-bonding interactions, with O···O hydrogen bond distances in the range 2.46−2.58 Å ( Figure S20). The crystalline structure is connected in a 2D framework with no interpenetration between adjacent layers (Figure 3d Tables S7 and S8). In contrast to MUV-20a and MUV-20b, the structure of MUV-21 contains dimethylammonium cations (DMA + ) originating from DMF decomposition.
The calculated free space of MUV-20a, MUV-20b, and MUV-21 is 22% (308 Å 3 as void volume), 15% (250 Å 3 ), and 27.6% (788.3 Å 3 ), respectively (see Figures S9, S17, and S24). However, despite the apparent large porosity of MUV-21, this material collapses upon activation ( Figure S38), probably caused by the stronger interactions of DMF molecules with the framework, as previously observed for analogous systems. 40 The crystallinity of MUV-20a and MUV-20b is maintained upon activation (1 h at 70°C), remaining stable up to 330°C (Figures S35−S37). CO 2 adsorption isotherms reveal a maximum CO 2 adsorption capacity, at 10°C and 6 bar, of 1.91 and 1.71 mmol g −1 for MUV-20a and MUV-20b, respectively (see Figure 4 and Table S9). These values are comparable to other HOFs found in the literature, such as HOF-6 43 and HOF-7, 44 although it is still far from the CO 2 record uptake of 4.02 mmol g −1 of HOF-5 at 1 bar and 296 K. 22 Comparing the two families of HOFs, MUV-21 has a higher void space and slightly shorter mean distances between TTF units than both MUV-20a and MUV-20b, which seems to indicate a better material for combining porosity and electrical conductivity, although, as already shown, the stability of MUV-21 upon activation is limited. However, close inspection of the structures reveals more relevant differences between the two families. Thus, whereas a DMA + cation is present in MUV-21 to counterbalance the negative charge of the deprotonated carboxylate, no additional cation is present in MUV-20a or MUV-20b. EPR measurements actually evidence the presence of an unpaired electron in both MUV-20a and MUV-20b, which is neither present in the as-synthesized NaH 3 TTFTB nor in MUV-21 ( Figure 5). The g values (g = 2.07) are in good agreement with an oxidized tetrathiafulvalene moiety. Thus, we hypothesize that compounds MUV-20a and MUV-20b can be considered as zwitterionic HOFs formed by molecules with a negative charge at one carboxylate unit and a positive charge at the TTF core. Unlike DMF, both THF and diethyl ether solvent molecules have a tendency to form peroxides, 45−47 which we believe is at the origin of TTF spontaneous oxidation.
In order to confirm the zwitterionic-radical nature of MUV-20a and MUV-20b, compared to MUV-21, theoretical calculations were performed under the density functional theory (DFT) framework using periodic boundary conditions. The minimum-energy crystal structure of the three compounds was obtained upon full lattice and ionic relaxation at the PBEsol level starting from the experimental X-ray data (see Section S9 in the Supporting Information for computational details). The optimized lattice parameters of the HOFs compare well with the values inferred for the synthetized crystals (see Table S13). Importantly, the presence of DMA + cations in MUV-21 leads to a H-bonding connection between the countercation and two TTFTB units (one of them with a deprotonated carboxylic acid; COO − ), involving two diagonal carboxylic positions of the ligands, whereas a H-bond network connecting four TTFTBs is found for the other two diagonal   Figure S48). In contrast, the frameworks of MUV-20a and MUV-20b can grow without any need for countercations toward the formation of a HOF with singly deprotonated TTFTB ligands and overall charge neutrality. The most stable site for the deprotonated carboxylic acid of TTFTB was assessed by placing it at the four inequivalent acid groups of the unit cell of MUV-20a and MUV-20b and fully relaxing the resulting structure ( Figure S49). Theoretical calculations predict the preferentially deprotonated carboxylic group as that showing short O(COO)···S(TTF) contacts in the range 2.7−3.1 Å for MUV-20a and MUV-20b ( Figure  S49). As experimental pieces of evidence indicate that the TTFTB ligand is singly deprotonated upon formation of the porous frameworks, the two ligands of the unit cell were considered to be singly deprotonated in their most stable isomer. 48 Moreover, as no countercation is present in the neutral crystal structure of MUV-20a and MUV20b, the negative charge of the carboxylate group has to be compensated by the formation of TTF •+ through spontaneous oxidation of the electron-donor moiety. We then propose two possible scenarios: (i) the extra electron of the carboxylate moves to the cationic TTF, leading to a carboxylic radical TTFTB species, or (ii) the ligand remains in its zwitterionic form engendering persistent charge carriers and a radical in the TTF core (Figure 6a). Preliminary calculations at the molecular level indicate that although both situations are computed very similar in energy, the zwitterionic species is more stable than the radical COO • analogue in singly deprotonated, neutral TTFTB when including boundary conditions ( Figures S50 and S51).
MUV-20a and MUV-20b can be modeled in a ferromagnetic or an antiferromagnetic configuration by considering the interaction between neighboring radical TTFTB ligands ( Figure S52). Theoretical calculations predict that the ferromagnetic alignment is 0.33 and 0.17 eV more stable in MUV-20a and MUV-20b, respectively, than the antiferromagnetic phase at the HSE06 level. In fact, the antiferromagnetic solution leads to the coupling of the unpaired electrons of the interacting TTFTB radicals, prompting a "diamagnetic"-like configuration (see Figures S53 and S54). As experimental data confirm the presence of the TTF radical, and following the theoretical relative stability calculations indicating that the ferromagnetic configuration is more stable, from now on, we focus on the ferromagnetic phase of both MUV-20a and MUV-20b.
Analysis of the electronic band structure and density of states (DOS) reveals a band gap of 1.42 eV in HOF MUV-21 ( Figure S55), with a valence-band maximum (VBM) centered on the TTF unit and a conduction-band minimum (CBM) localized over the benzoic acid groups (Figure 6b). On the other hand, the band gap in MUV-20a and MUV-20b is predicted to be 1.68 and 1.52 eV, respectively, in the β-spin manifold (Figures 6c and S56, respectively). In this case, both the VBM(α) and CBM(β) are centered over the TTF moiety (Figures 6b, S57, and S58), suggesting the formation of the radical TTF •+ . The spin density calculated for MUV-20a ( Figure 6d) and MUV-20b ( Figure S59) is fully located over the TTF core, which experiences a charge accumulation increase (Δq) of +0.58 and +0.55e, respectively, with respect to the isolated, fully protonated TTFTB ligand, whereas the deprotonated carboxylic group is predicted with a Δq of −0.70 and −0.66e, respectively. These results evidence the presence of an oxidized TTF core (with an unpaired electron; TTF •+ ) and a negatively charged carboxylate group (COO − ) per TTFTB ligand, therefore confirming the zwitterionic nature of MUV-20a and MUV-20b (Figure 6a).
In our HOFs, the TTF units are assembled in a π-stacking arrangement along the a-axis; thus, charge conduction after carrier generation may arise anisotropically in one dimension. The electronic band structure calculated for our series of HOFs displays flat bands along the full k-path of the first Brillouin zone (Figures 6c, S53, and S56 for MUV-20a, MUV-20b, and MUV-21, respectively), which points toward a hopping regime as the dominant charge-transport mechanism. The size of the polaron was estimated from the spin density contours to localize over few (1−3) TTF units in the three HOFs ( Figure S60), 49 supporting a hopping-like transport.
To shed light on the conducting properties of MUV-20a, MUV-20b, and MUV-21, the electronic couplings (J) between neighboring TTFTB ligands, the reorganization energy, and the resulting charge-transfer rate constants (k) were estimated by means of the Marcus theory and molecular calculations (see Section S9 in the Supporting Information for details). The energy splitting between the highest-occupied crystal orbitals involving the two TTFs of the unit cell (VBM energy splitting) 50 was evaluated as a bare approximation of the electronic communication between TTF pairs, showing a larger mean value for zwitterionic MUV-20a (175 meV) and MUV-20b (158 meV) than for MUV-21 (90 meV). Accurate electronic couplings for the closest interacting TTF pairs (dimer A in Figure 7) confirm this picture, with J values of 152, 92, and 55 meV for MUV-20a, MUV-20b, and MUV-21, respectively, which are in the same order of magnitude of those reported for prototypical organic semiconductors. 51,52 The coupling in the weakly interacting TTF pair (dimer B) is predicted one order of magnitude smaller (14, 37, and 7 meV for MUV-20a, MUV-20b, and MUV-21, respectively), which stems from the less efficient π-stacking of the TTF moieties (core-to-core intermolecular distances > 5 Å; Figure 7). Overall, considering the smaller electronic coupling as the limiting step for charge conduction, we predict rate constants   (Table S14). As conductivity depends on charge mobility and carrier concentration, zwitterionic MUV-20a and MUV-20b, which present larger TTFTB electronic couplings and already contain the TTF •+ / COO − polaron in the framework, are expected to show a significant enhancement of charge transport compared to bare MUV-21.
Transport measurements for MUV-20a, MUV-20b, and MUV-21 were performed using two-contact probe pressedpellet devices measured at room temperature (300 K) ( Figure  S46), revealing conductivity values of 6.07 × 10 −7 , 1.35 × 10 −6 , and 6.23 × 10 −9 S cm −1 , respectively (see Table S11). This indicates that MUV-20a and MUV-20b behave as semiconductors, presenting electrical conductivities similar to those reported for Cd 2 (TTFTB) 53 and TTF-COF 39 after being oxidized with iodine and more than 10 times higher than iodine-doped HOF-110 42 (see Table S12 for comparative values with all reported MOFs and HOFs containing the TTFTB unit). In contrast, non-zwitterionic MUV-21 behaves as an insulator due to the absence of charge carriers. Thus, conductivity measurements confirm the semiconducting nature of zwitterionic MUV-20a and MUV-20b, which present record charge transport in HOFs without the need for further oxidation due to the existence of persistent positive charge carriers in the form of TTF •+ radicals.
Due to the fast synthesis of the crystals under ambient conditions, the processability to prepare thin films of these porous semiconductor materials was examined for MUV-20a. Drop-casting of a 0.02 M THF solution of NaH 3 TTFTB on a 3 × 3 cm 2 glass support yields a continuous thin film, as shown in Figure 8. This illustrates the facile processability of HOFs, contrary to what is commonly found for MOFs and COFs, which paves the way to their implementation in charge-storage devices, electrochemical sensors, or as electrocatalysts.

■ CONCLUSIONS
A new strategy for the synthesis of conducting HOFs is presented, based on the in situ oxidation of a partially deprotonated TTF-based molecule, which yields a zwitterionic material. This has been exemplified with two different structures, namely, MUV-20a and MUV-20b, which are obtained using the TTFTB building block and have different solvents in the pores (THF and ether, respectively). On the contrary, the solvothermal approach in DMF commonly used for the preparation of porous materials yields a nonzwitterionic HOF, denoted MUV-21. Whereas MUV-21 collapses during pore activation, MUV-20a and MUV-20b display high stability upon evacuation, likely due to their zwitterionic behavior and lower pore sizes, and offer large CO 2 sorption capacities up to 1.91 and 1.71 mmol g −1 , respectively. Theoretical calculations confirm the zwitterionic nature of these two HOFs and the emergence of efficient charge transport properties without the need of postsynthetic oxidation treatments. Transport measurements indicate conductivity values of 6.07 × 10 −7 and 1.35 × 10 −6 S·cm −1 for MUV-20a and MUV-20b, respectively, thus placing these materials as the highest conducting HOFs so far, with the added value of facile synthesis for further application in organic electronics.

■ EXPERIMENTAL SECTION
H 4 TTFTB was prepared as previously described, 38,54 and all details can be found in the Supporting Information.
Crystallization of MUV-20a. 2 mL of tetrahydrofuran (THF) was added to 20 mg of NaH 3 TTFTB in a 40 mL vial. The mixture was sonicated for 2 min until a transparent dark-red solution was formed. This solution was heated for 5 min at 80°C by placing the vial on top of a heating plate. Then, 30 mL of diethyl ether was added, resulting in red needle-like single crystals of MUV-20a, after few minutes at room temperature, in which THF molecules are embedded in the pores ( Figure S11). Finally, the crystals were filtered in air (12 mg, yield 60%).
Crystallization of MUV-20b. Upon formation of red needle-like single crystals of MUV-20a, these were washed with diethyl ether, yielding red needle-like single crystals of MUV-20b (16 mg, yield 80%), in which THF molecules are replaced by diethyl ether molecules ( Figure S19). In fact, immersion of crystals of MUV-20a in diethyl ether causes the transformation of MUV-20a to MUV-20b (see Figure S31 in the Supporting Information).
Crystallization of MUV-21. 20 mg of NaH 3 TTFTB was solved with 1 mL of DMF in a 4 mL vial at 105°C for 60 h in an oven with heating and cooling ramps of 0.5°C min −1 , resulting in red needlelike single crystals of MUV-21 that were washed with diethyl ether at room temperature (4 mg, yield: 20%).
Single-Crystal Diffraction. X-ray data for compound MUV-20a were collected at beamline I19 in a Diamond Light Source Journal of the American Chemical Society pubs.acs.org/JACS Article synchrotron at a temperature of 100 K using a Pilatus 6M detector. 55 X-ray data for the compound MUV-20b were collected at 100 K using a Rigaku FR-X rotating anode with a Hypix 6000HE detector. X-ray data for compound MUV-21 were collected at 120 K using a Rigaku microfocus Supernova with an Atlas CCD detector. Data were measured using GDA and CrysAlisPro suite of programs. X-ray data were processed and reduced using the CrysAlisPro suite of programs. Absorption correction was performed using empirical methods (SCALE3 ABSPACK) based upon symmetry-equivalent reflections combined with measurements at different azimuthal angles. The crystal structures were solved and refined against all F 2 values using the SHELX and Olex 2 suite of programs. 56 Electron Paramagnetic Resonance. Electron paramagnetic resonance was measured using a Bruker ELEXYS E580 at 300 K.
Theoretical Calculations. Quantum chemical calculations were carried out within the DFT framework as implemented in the allelectron full-potential FHI-AIMS electronic structure package. Minimum-energy geometries for MUV-20a, MUV-20b, and MUV-21 were obtained starting from the experimental crystal structures, and after full lattice and ionic relaxation using the GGA-type PBEsol functional and the numeric atom-centered orbital tier-1 basis set. Dispersion forces were treated by means of the van der Waals Hirshfeld correction. The electronic band structure and density of states were obtained by means of the hybrid HSE06 functional. A full k-path in the P1̅ first Brillouin zone and a 3 × 3 × 3 k-grid were employed. The electronic couplings of the different TTF dimers were computed under the fragment-orbital DFT framework, as implemented in FHI-AIMS. See the Supporting Information for full computational details.
Conductivity Measurements. Powders of MUV-20a, MUV-20b, and MUV-21 were pressed to form pellets (P ≈ 5 US tons), cut in rectangular shapes, and allowed to make contact with platinum wires (Goodfellow, 99.99%, 25 μm of diameter) and silver conductive paint (RS 123-9911) in a two-probe configuration ( Figure S46). Two different pellets were measured for each sample. The geometrical factors (width, length, and thickness) were measured using an optical microscope. MUV-21 was also measured using a single crystal ( Figure  S46).
Film Recrystallization. For the synthesis of the film, 40 mg of NaH 3 TTFTB was dissolved in 2 mL of THF and then was drop-cast on a 3 × 3 cm 2 glass. After a few minutes, the solution was dried and a continuous film forming dark small crystals appeared on the surface of the support.
General methods, materials, crystallographic data, computational details, and supporting theoretical data (PDF)